The present invention relates to a stacker control.
Shelf stackers (special sorts of fork lift trucks, the fork of which can be lifted and lowered vertically but also have additional fork functions such as additional lift, thrust, turning the fork), in particular stackers for high storage racks, comprising swing-shift forks require a high degree of safety during control of the fork if they operate within a laterally defined aisle area, e.g. between rows of shelves. For reasons of space the aisle area is often so limited that uncontrolled turn and thrust movements of the fork may easily lead to collisions. Therefore, for such complicated maneuvering operations, shelf stackers are often operable by means of a computer which guides the stacker control by simultaneously swinging and shifting the fork, so that the fork does not collide with the rows of shelves. To this end, the computer must not only be informed in a precise manner about the actual position of the fork, but the stacker control must be precisely guidable by the computer, i.e. exactly in proportion with the control signals. A further problem encountered in the case of a precise proportional control of the swing-shift fork arises from the fact that hydraulic motors which carry out swinging and shifting operations, respectively, optionally at the same time, require clearly distinct flow rates, e.g. in a ratio of 1:3 to 1:6, and the control of the hyraulic motor with the smaller rate must also be reliably proportional in cases where the hydraulic motor requiring the larger amount is moved at the same time.
It is known in practice that in the stacker control of a shelf stacker a three-way flow regulator (3-way control valve) and a proportional directional control valve with upstream supply regulator are assigned to each hydraulic motor to achieve precise proportionality during control, as is e.g. required for maneuvering operations within narrow aisle areas. Such a stacker control is not only very troublesome and expensive, but requires a lot of space, which is not readily available in the case of shelf stackers, because of the complicated hydraulic components. Therefore, such a stacker control is also very troublesome for the reason that a specific neutral circulation switching function of the hydraulic medium, which is delivered by the source of pressure, is required, for instance in case of pilot pressure tapping (PSL load sensing principle), in the directional control valves.
Furthermore, it is known in practice that the stacker control of such shelf stackers is equipped with a plurality of pressure balances, directional control valves and lowering brake valves (drop-rate braking valves) to save space and costs. However, it is thereby not possible to achieve a sufficiently proportional control, as is e.g. needed for maneuvering operations within limited aisle areas. Such shelf stackers first must move out of the aisle area for the swing-shift adjustment of the fork, and must again move into the aisle with the adjusted fork, or the aisle areas between rows of shelves must be made correspondingly wide in relation with the width of the fork.
In control devices which are known from WO86/06142, U.S. Pat. No. 3,911,942, U.S. Pat. No. 3,987,623 and DE-A-1 95 49 150, control priority is given to a selected group of consumers over other consumers. The control devices contain three-way pressure balances or multi-way valves which serve flow control purposes.
Further prior art is found in U.S. Pat. No. 4,517,800, U.S. Pat. No. 4,733,533, DE-A-301 61 57 and U.S. Pat. No. 4,543,031.